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Some linux distros have packages for installing this, but not all ...
Installing Oracle XE on linux (manually)
Oracle XE is an entry level database free of charge but with embeded limitations: 
- will use only CPU resources that are equivalent of 1 CPU/core on your system
- will use only up to 1 Gb of RAM
- is limited to 1 server/database instance per machine
- is limited to 11G of user-space data
Follow the installation procedure is described in the XE installation manual.
If you use a distro like Ubuntu a good manual for most cases (not only Ubuntu 11.10) can be found at oracle forum
If you're installing this DB for occasional use MAKE SURE you set it up so it will not start at boot. This DB goes through resources like the Cookie Monster through a Bahlsen factory
Some basic DBA commands can be found on XE DBA Documentation page but in most cases you can use DB control interface that can be accessed trough DB's http port (the one you selected when you ran "oracle-xe configure")
Installing Oracle Instant Client on linux (manually)
Follow the instructions found at Oracle's PHP documentation page.
If you are not using the "hostname/servicename" connection string format (e.g. "localhost/xe"), and instead use an alias from a tnsnames.ora file, then after you have your PHP environment set up add the TNS_ADMIN variable to your PHP startup environment. This variable must contain the location (directory only) where your tnsnames.ora file is located. This file contains TNS definitions of the databases you will be accessing (local or remote). If you have previously set up XE on this machine this variable should point to '/u01/app/oracle/product/11.2.0/xe/network/admin'. Make sure this file is readable by the Apache process.
After you have OCI8 enabled and TNS_ADMIN set in your PHP env you can now install MW normally. TNS name of default XE install is usually 'XE'.
Test drive your wiki ...